The disk drive industry is driven by competition in the market to provide ever increasing capacity which, because disk form factors are relatively fixed, results in increasing density of the data recorded on the disk. This density increase may be achieved in two dimensions--circumferentially by packing transitions closer together within a track, and radially by packing more, narrower tracks across the disk. The industry presently prefers sampled track positioning systems where a transducer head mounted on an actuator reads positional information (servo) sectors which are interspersed with data sectors around a track on the disk. The industry also is presently transitioning from a single inductive transducer head to a dual element transducer using a magnetoresistive (MR) head for reading and an inductive head for writing. These disk drives present a particular design challenge in that the data sectors and servo sectors within a circumferential track require completely different processing. With narrower, more closely spaced tracks the problem of precisely maintaining the position of the head over a track is greatly exacerbated. The MR head, although an improvement for reading densely packed data, is particularly a problem in processing servo sectors because of non-linearity in reading magnetic bursts used for determining position and, because of the read and write heads being necessarily spaced apart from each other, there is an offset between them relative to a track passing under the heads which varies according to the radial position of the head over the disk.
Conventionally, a disk drive has a track structure as shown in FIG. 3. A plurality of radially concentric tracks 67 is disposed across a disk 12. The tracks 67 are spaced apart by a data track "pitch" 63 usually expressed in tracks per inch or the reciprocal. Positioned at regular intervals around the tracks are servo sectors 66 written by a servo track writer during manufacturing of the disk drive. When viewed radially across the disk, servo sectors 66 form "servo wedges" 64. The data areas between the servo wedges 64 may similarly be termed "data wedges" 62 having data sectors 65.
As shown in FIG. 1, servo sector 66 comprises a Preamble/Servo Sync Word (SSW) field 102 and a track identification field 104 followed by a group of servo bursts 130,132,134,136. A track positioning servo system (not shown) uses the information from track identification field 104 and servo bursts 130,132,134 and 136 to derive a position error signal (PES) which is minimized by a servo controller to place transducer 120 over a data sector 65 (shown in FIG. 3) in track 67 for reading or writing. Transducer 120 is placed approximately on track center using the track identification field 104 as a coarse position indicator during a seek operation. The transducer is then precisely positioned at track center by reading the servo bursts 130,132,134,136 and correcting the position of the actuator accordingly.
Conventionally, the servo controller finds the center of a track when reading equal portions of servo burst pairs, such as bursts 134 and 136, whose adjoining edges 101 coincide with the track center. Such adjoining edges 101 may be termed a "burst pair centerline." Additional bursts 130 and 132 are provided to resolve ambiguity caused by so-called "blind spots" when the head is unable to resolve burst amplitude differences for incremental position changes. In this specification the space between burst pair centerlines 101 on adjacent tracks forms a "servo track pitch" 61 and in the prior art is coincident with the data track pitch 63. This arrangement disadvantageously constrains the designer's choice of servo track pitch, which is driven by the requirements of a servo control channel and by the economics of servowriting during manufacture, to be bounded by data track pitch which may be driven by other factors such as data density, channel bandwidth, drive mechanics, and competitive market factors. Thus a need is felt for a disk design methodology which allows a designer to separately optimize data tracks and servo tracks.